Having Big Genitals Drove a Species Extinct

In defense of being average.

McDonald’s fast food servers, Cadillac, and ‘80s hairstylists have made the compelling case that bigger is always better. But in the biological world, that isn’t always true. This year, researchers uncovered evidence of some unfortunate species that met their demise because their subscribed to that mantra. Their genitals, way larger than those of its relatives, took so much energy to build that they didn’t have any evolutionary juice left to survive.

The paper, published in Nature in April, focused on a little-known tiny crustacean called an ostracod. Some species of this animal are still around today, but the fossil evidence, spanning 93 species of ostracods that lived between 85 and 65 million years ago, shows that many other species have gone extinct. What so many of those long-gone species had in common was that the males all had (relatively) huge genitals.

“We showed that when males are larger and more elongated than the females, those species tend to not last as long in the fossil record,” study co-author and paleobiologist Gene Hunt, Ph.D., said at the time. “They have a higher risk of extinction.”

Female (top) and male (below) of the ostracod *Cypideis salebrosa*. The genitalia is highlighted in blue.

The explanation Hunt and his team offered seemed counterintuitive to the long-standing biological concept that bigger is better when it comes to genitals. So much of a species’ evolution has to do with the ability of any individual to pass on its genes through successful mating, and many animals are thought to have developed bigger and more elaborate genitals in order to maximize mating success with the opposite sex.

In plain terms, the theory suggests that many male animals have large (often bone-filled) penises to make sure it can stay in a female’s vagina long enough to inseminate her. Sometimes — we’re looking at you, ducks — the male penis even takes on a crazy shape like a corkscrew. (But it’s worth noting that in some unusual species, the sexual dynamic is flipped: The female develops a “penis” to ensure mating with the male “vagina.”)

But the extinct ostracods suggest that there’s an energetic cost to developing such huge genitals.

“If devoting so much energy to reproduction made it harder for species in the past to adapt to changing circumstances,” said Hunt, “perhaps that same should apply to species we’re concerned about conserving in the present day.”

An extensive survey of fossilized ostracods showed that those with bigger genitalia tended to go extinct faster than their average-sized counterparts. Nice!

In other words, there is more to life than trying to grow huge genitalia. If you’re an ostracod, that could mean that the energy that might’ve gone into giving you a size boost might have been better used to acquire food, care for your children, defend against predators, or cope with a changing climate — all factors also involved in survival. Word is out for whether the same goes for humans too.